Laser Assisted SiC Growth On Silicon

ABSTRACT

A method for forming a compound on a substrate is provided. The method includes depositing a composition onto a surface of a substrate; illuminating the composition and the substrate with pulsed energy; melting the substrate and decomposing the composition simultaneously; and forming a compound on the substrate. A first component of the compound is derived from the substrate and a second component of the compound is derived from the composition.

GOVERNMENT RIGHTS

This invention was made with government support under N6601-12-1-4238awarded by the U.S. Defense Advanced Research Projects Agency. Thegovernment has certain rights in the invention.

FIELD

The present disclosure relates to methods for making heterojunctions forphotovoltaic and other optical devices, and high power and highfrequency devices.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

According to a United Nations (UN) report, with current energy usage(largely based on fossil fuels), the Earth's temperature will risebetween 1.5 to 4° C. over the next 50 years due to greenhouse gases.With this rise in temperature, Earth faces a future of extreme weather,rising sea levels and melting polar ice from soaring levels of carbondioxide, methane and other greenhouse gases. A global effort in greenmanufacturing and use of renewable energy resources is necessary toreduce emission to prevent temperatures from rising; preventingcatastrophic and dangerous disruptions worldwide. In parallel to thischallenge, a quarter of the world's population (roughly 1.6 billion) hasno electricity. Lack of availability of electricity has a directcorrelation to poverty and poor health. The current approach oftransporting energy to remote locations is difficult and expensive.Thus, production of energy from local resources is necessary. Use ofphotovoltaics (solar cells) and their manufacturing play an importantrole in the improvement of environment and society.

Photovoltaic (PV) devices directly convert sunlight into electricity. Onaverage, the sun illuminates the Earth with more than 10,000 times thelight energy humans currently consume, PVs have the potential to be alarge and environmental friendly energy source. The production cost ofsolar electricity has reduced over the last decade and can now competewith grid electricity. This increase is driven by advances in PVtechnology, large scale manufacturing, and also because the relativecost of oil and coal has increased over the last decade. Crystallinesilicon (c-Si) based PVs currently dominate the market due to theirlarge efficiency and low cost of production. This domination is alsolargely because Si is available in abundance, it is non-toxic and it isstable under harsh environment conditions. Further advances in thistechnology are needed to make it compatible with green manufacturing,achieve higher efficiency and to reduce overall cost.

One of the biggest obstacles to harvesting renewable energy sources iscost. Many approaches to harvesting solar energy are being explored bythe scientific community (e.g., solar cells, thermal concentrators,etc.). Among the many viable approaches, solar cells are attractive asthey provide direct conversion from light to electric energy. Bothorganic and inorganic materials are being explored for the design ofhigh efficiency low-cost solar cells. Organic solar cells, althoughhaving the potential to reduce cost, still suffer from long termstability. Inorganic materials for solar cell production, the mostcommonly used being Gallium Arsenide (GaAs), silicon (Si), IndiumPhosphide (InP), and cadmium telluride, have shown to have long termstability while still maintaining the potential to reduce cost. Reducingthe cost of producing solar electricity plays an important role inspreading its use as a clean and renewable energy source. The cost tomanufacture solar cells can be reduced in two ways, by increasing theirefficiency and by developing low-cost production technologies. Amongthese device technologies, silicon (Si) devices are attractive as thebase material. Silicon is available in abundance in nature andthin-films of Si can be deposited using a variety of techniques, such ase-beam, plasma enhanced chemical vapor deposition, and pulsed laserdeposition. Silicon based technologies provide an optimum balancebetween material cost, efficiency and product life time.

The reason Si solar cells have not become popular in daily life isbecause existing solar cell processes are very elaborate and requireexpensive fabrication steps. The number of mask steps required tofabricate solar cells can be large and minimizing these steps isnecessary to reduce cost. Furthermore, for high efficiency solar celltechnologies, the cost associated with the manufacture of Si wafers ishigh due to required chemical and mechanical polishing. Among the manytypes of solar cell device technologies, silicon heterojunction (SHJ)solar cells have become popular due to their large efficiencies andability to process at low temperatures at reduced cost. FIG. 1 shows across sectional diagram of a SHJ solar cell. The SHJ consists of thinhydrogenated amorphous silicon (a-Si:H) wide bandgap buffer layersdeposited on crystalline silicone (c-Si) wafers. These hydrogenatedbuffer layers (a-Si:H) help improve the quality of the material andprovide a higher bandgap layer. A transparent conductive oxide (TCO)layer is deposited on the highly doped a-Si:H layers. Top and bottomelectrode layers are screen printed. This design enables energyconversion efficiency above 20% at the production level. The key featureof this technology is that the metal contacts, which have a highlyrecombination active interface, are separated from the absorption regionby utilizing wide bandgap a-Si:H layers. This enables high open-circuitvoltages typically associated with heterojunction devices. Pyramidstructures (5-10 μm) are included to enhance capture of incoming light(anti-reflection, AR, layer). The heterojunction solar cells are alsoutilized to enhance the absorption of a wider optical spectrum. Theefficiency of Si solar cells increases by growing a top layer of asemiconductor (a-Si:H) with an energy bandgap larger than the c-Silayer. Incident photons with energy greater than the bandgap energy ofa-Si:H will be absorbed in this top layer. Photons of lower energy willbe absorbed in the c-Si layer to form carrier pairs. In this way, moreof the energy of the solar spectrum is used to generate electricalpower.

From a processing point of view, the key advantages of SHJ technology isthat thin Si wafers can be used and an overall fewer number offabrication steps are required compared to other Si based devices. Thepyramidal texture is formed using a mixture of potassium hydroxide (KOH)and isopropyl alcohol (IPA) solutions at an elevated temperature.Texturing helps by lowering external optical reflection, and inparallel, improves internal reflection, which improves light trapping.Such texturing can be formed on polished single-crystal silicon, thoughin recent years methods of forming them on multi-crystalline siliconhave been developed. The a-Si:H layers are deposited in vacuum usingchemical vapor deposition using silane gas. Prior to the deposition ofa-Si:H layers, the c-Si is cleaned to minimize interfacial defects.Oxide and nitride layers are deposited to passivate the surface as apassivation layer. This passivation layer is patterned prior to screenprinting conductive electrodes on both sides. Although the above solarcell design minimizes many process steps compared to conventionalsilicon solar cells while achieving high efficiency, the expensive stepsremaining are the substrate material (c-Si), vacuum deposition of a-Si:Hlayers, and wet etching of textured surface. Thus, there is a need todevelop a manufacturing process that: i) avoids the use of vacuumdeposition, ii) does not require polished Si substrates, iii) avoids theuse of corrosive chemicals to achieve antireflection (AR) texturing, andiv) further enhances conversion efficiency. Here, a silicon carbide(SiC) based heterojunction device processed using a laser as an approachto meet the above challenges is provided.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The present technology provides a method for forming a compound on asubstrate is provided. The method includes depositing a composition ontoa surface of a substrate; illuminating the composition and the substratewith pulsed energy; melting the substrate and decomposing thecomposition simultaneously; and forming a compound on the substrate. Afirst component of the compound is derived from the substrate and asecond component of the compound is derived from the composition.

The present technology also provides a method for forming SiC on asilicon substrate. The method includes depositing a carbon source on asurface of a Si substrate; illuminating the carbon source and Si with anexcimer laser that generates from about 200 mJ to about 1000 mJ ofenergy with pulses of from about 20 ns to about 1000 ns; moving theexcimer laser relative to the Si substrate and carbon source; andforming SiC on the Si substrate. Fabrication can take place on any typeof Si substrate, including polished, non-polished, amorphous, powdercoated, and pre-processed wafers.

Also, the present technology provides a heterojunction device. Theheterojunction device includes a Si substrate and a film of SiCdeposited on a surface of the Si substrate. The SiC has a Si:C ratiothat increases or decreases from a SiC surface in contact with the Sisubstrate to an opposing SiC surface that is not in contact with the Sisubstrate.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic illustration of a silicon heterojunction cellalong with a band diagram;

FIG. 2 is a diagram showing a method for forming a compound on asubstrate according to the present technology;

FIG. 3A is a schematic illustration of a double-sided heterojunctiondeveloped according to the current technology;

FIG. 3B is a simplified energy band diagram of the double-sidedheterojunction shown in FIG. 3A;

FIG. 4A is a schematic illustration of fabrication steps of laserprocessed SiC-Si heterojunction devices;

FIG. 4B is a photomicrograph of an exemplary fabricated device;

FIG. 5 shows a band diagram of a fabricated diode;

FIG. 6A shows Raman spectra of a SiC film on a Si substrate, wherein theinset is a photomicrograph of the film on the substrate;

FIG. 6B shows an exploded view of the Raman spectra shown in FIG. 6A;

FIG. 7 is a graph showing measured J-V characteristics of a Si-SiCheterojunction fabricated using different number of pulses with highbreakdown voltage;

FIG. 8 is a graph that shows measured C-V characteristics of Si-SiCheterojunction, wherein a straight line is included for visualreference;

FIG. 9A is a graph showing measured J-V characteristics of a SiC/Siheterojunction solar cell for different number of pulses;

FIG. 9B is a graph showing dark and illuminated J-V curves for a singlepulse device with current density (J_(sc))=17 mA/cm², open circuitvoltage (V_(oc))=0.33V and fill factor of 62%;

FIG. 9C is a graph showing internal quantum efficiency (IQE) spectra.

FIG. 10A is a scanning electron micrograph (SEM) of a SiC layer treatedwith multiple fixed power laser pulses;

FIG. 10B shows photomicrographs of devices generated from differentnumber of pulses;

FIG. 11A is a schematic representation of a highly doped SiC top layersurrounded by hexagonal carbon conductor lines;

FIG. 11B is a photomicrograph showing carbon contact formation on theperimeter of a device;

FIG. 12 is a photomicrograph of a device fabricated using a laser on anunpolished Si wafer (4 pulses);

FIG. 13A is a graph showing J-V characteristics of devices on unpolishedwafers under dark and illuminated condition for a single pulse device;

FIG. 13B is a graph showing measured J-V under illumination conditionsfor devices fabricated using differing numbers of laser pulses;

FIG. 14 is a schematic illustration of fabrication steps for SiC/Sidiodes for radio frequency (RF) circuits, wherein panel I shows growthof SiC using an excimer laser, panel II shows a first metal layer(Ni/AI) coating, panel III shows Ni/AI patterning and SiC plasmaetching, panel IV shows plasma enhanced chemical vapor deposition(PECVD) of SiO₂, panel V shows etching oxide using buffer oxide etch(BOE) (1:6), and panel VI shows top Al deposition and patterning;

FIG. 15 is an optical micrograph of a fabricated coplanar waveguide(CPW) structure with a SiC/Si diode;

FIG. 16 is a graph showing measured Raman spectra of SiC/Si for a devicemade with 2 laser pulses, wherein the optical image of the fabricateddevice is provided inset;

FIG. 17 is a graph showing measured J-V characteristics of SiC/Si baseddiodes (Type A) fabricated on low doped wafer, wherein the inset shows acurve fit to a diode equation;

FIG. 18 is a graph showing measured J-V characteristics of SiC/Si baseddiodes (Type B) fabricated on a high doped wafer, wherein the insetshows a curve fit to a diode equation;

FIG. 19 is a graph of rectified current versus input signal frequencyfor SiC/Si radio frequency (RF) diodes made on low doped wafers (diodetype A) and at a fixed bias of about 0.35 V and RF power of about 4 dBm,wherein the inset shows rectified output signal as a function of DC biasat 5 and 6 GHz and of about 4 dBm input RF power;

FIG. 20 is a graph of Rectified current vs input signal frequency forSiC/Si RF diodes made on high doped wafers (diode type B) and at a fixedbias of about 0.35 V and RF power of about 4 dBm, wherein the insetshows rectified output signal as a function of DC bias at 5 and 6 GHzand of about 4 dBm input RF power;

FIG. 21 is a graph showing measured rectified current at fixed bias(0.35 V) versus input power for SiC/Si radio frequency (RF) diodes at 3,5 and 6 GHz;

FIG. 22 is a graph showing measured output power of second harmonicversus fundamental frequency of a SiC/Si (type II) diode at an inputpower of approximately −3 dBm; and

FIG. 23 is a graph showing measured output power of second harmonic forSiC/Si at fundamental frequencies of 2 and 4 GHz.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentiallyof.” Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of,” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Although the terms first, second, third, etc. may be used herein todescribe various steps, elements, components, regions, layers and/orsections, these steps, elements, components, regions, layers and/orsections should not be limited by these terms, unless otherwiseindicated. These terms may be only used to distinguish one step,element, component, region, layer or section from another step, element,component, region, layer or section. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first step,element, component, region, layer or section discussed below could betermed a second step, element, component, region, layer or sectionwithout departing from the teachings of the example embodiments.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. Other than in the workingexamples provided at the end of the detailed description, all numericalvalues of parameters (e.g., of quantities or conditions) in thisspecification, including the appended claims, are to be understood asbeing modified in all instances by the term “about” whether or not“about” actually appears before the numerical value. “About” indicatesthat the stated numerical value allows some slight imprecision (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If the imprecision provided by “about” isnot otherwise understood in the art with this ordinary meaning, then“about” as used herein indicates at least variations that may arise fromordinary methods of measuring and using such parameters.

As referred to herein, ranges are, unless specified otherwise, inclusiveof endpoints and include disclosure of all distinct values and furtherdivided ranges within the entire range. Thus, for example, a range of“from A to B” or “from about A to about B” is inclusive of A and of B.Disclosure of values and ranges of values for specific parameters (suchas temperatures, molecular weights, weight percentages, etc.) are notexclusive of other values and ranges of values useful herein. It isenvisioned that two or more specific exemplified values for a givenparameter may define endpoints for a range of values that may be claimedfor the parameter. For example, if Parameter X is exemplified herein tohave value A and also exemplified to have value Z, it is envisioned thatParameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if Parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, and 3-9.

The current technology provides apparatuses and methods directed tophotovoltaic devices, light harvesting systems, and microwave circuits.The methods avoid the use of vacuum deposition, do not require polishedSi substrates, avoid the use of corrosive chemicals to achieve ARtexturing, and further enhance conversion efficiency. In variousembodiments, the methods include depositing a composition onto a surfaceof a substrate, illuminating the composition and the substrate with apulsed energy source, melting the substrate and decomposing thecomposition, and forming a compound on the substrate. A first componentof the compound is derived from the substrate and a second component ofthe compound is derived from the composition.

From a manufacturing point of view, the current method avoids theexpensive steps typically used in the fabrication of Si solar cells,diodes, etc. Overall, the energy and material costs required tofabricate devices according to the method are very low. The moderateenergy and material demand for the fabrication of the solar cells,diodes, etc. will lead to short energy payback times. Advantagesassociated with the current method include: i) solar cells, diodes, etc.can be fabricated at atmospheric pressure (without resorting to ahigh-cost vacuum process as conventionally used) using an excimer laserprocess; ii) solar cells, diodes, etc. can be fabricated on as-cutunpolished low-cost Si wafers; iii) energy bandgap tailored SiC can bedirectly formed on Si layer through laser processing; iv) during a lasersintering of the current method, the surface can also be textured toenhance light trapping; and v) carbon interconnect layers can be formedusing the same laser setup required by the method through pyrolysis bylowering the laser power where interconnects need to be formed.

Beyond the above advantage points, the process can further be advancedto grow a thin Si layer directly from Si powder on conductive flexsubstrates (e.g., steel foil) prior to the formation of a SiC layer. Theprocess is versatile and allows the growth of solar cells or diodes on3D structures.

Apart from solar-cells, Si-SiC devices fabricated by the methodsprovided herein hold significant potential in a range of other devices,such as, for example, low-cost photodetectors, high-voltage converters,high power high frequency devices, high temperature circuits, opticaldiodes, water splitting, and others. The proposed technique also holdssignificant importance for many commercial and military applications byallowing the growth of solar-cells on 3D structures such as helmets,structures of a car, side panels of houses, and housing of hand-heldelectronic units. SiC is an excellent material for these applicationsdue to its wide bandgap, high thermal conductivity, high electronmobility, and chemical stability.

There has been a steady growing interest in wide bandgap silicon carbide(SiC) films for solar cell applications. SiC has a long history and ithas been commonly used as an abrasive material for over a century, andmore recently in optical devices such as LEDs, lasers and photodetectorsas well as high voltage high power devices. Similar to Si, it is anindirect gap semiconductor with a forbidden bandgap (Eg) in the range of2.38<Eg<3.26 eV (depending on the polytype). It can be converted into adirect bandgap by substituting carbon (C) with germanium (Ge) ornitrogen (N) atoms and has excellent chemical resistant properties toacids and bases. The native oxide of SiC is SiO₂, which is important inthe growth of electronic devices. It is commonly deposited usingchemical vapor deposition (CVD) largely on Si substrates. Also, therehas been a great amount of work done on the growth of SiC fromcrystalline Si through reaction with hydrocarbons at elevatedtemperatures under vacuum. Some of these processes have been adopted inthe fabrication of SiC solar cells. Similar to the structure of FIG. 1,c-Si solar cell with a hydrogenated amorphous silicon carbide (a-SiC:H)buffer layer in place of a-Si:H buffer layers has been demonstratedwhile achieving solar cells with efficiency near 20%. Here again, CVDwas used to grow an a-SiC:H layer and the remaining process steps arethe same as the SHJ device discussed above. Thus, from a manufacturingpoint of view, the benefit gained by using CVD grown a-SiC:H is notsignificant. However, if growth of SiC can take place at atmosphericpressure and through the use of simple fabrication steps then SiC holdspotential for low cost solar cell applications. Here we propose theformation of a SiC buffer layer using an excimer laser which can voidthe use of the CVD process and while also providing other benefits asdiscussed later which can help reduce the overall cost of manufacturingheterojunction solar cells. First, a brief background on excimer lasersis given.

Excimer Lasers

Excimer lasers are pulsed gas discharge lasers. They generate opticaloutput in the ultraviolet region of the spectrum. Excimer laserstypically operate at a wavelengths of 193 nm (ArF gas, 6.4 eV), 248 nm(KrF gas, 5.0 eV), 308 nm (XeCl gas, 4.0 eV), or 351 nm (XeF). Excimerlasers are highly precise and are commonly used in eye surgery,photolithography, deposition of materials, and micromachining. Theselasers have been used to transform thin layers of amorphous Si (50-100nm) into high quality polycrystalline Si, with enhanced electronmobility, for use in flat-panel displays for mobile phones andflat-screen televisions. The advantage of using a laser in the annealingand sintering of Si is that energy can be transferred efficiently over alocalized area with high resolution, and thin low cost substrates can beused. Excimer lasers are also commonly employed in the deposition ofthin films, including Si. The output power, reliability, duty cycle ofpulses and wavelength purity have been greatly enhanced over the lasttwo decades and thus have found great reception in the manufacturingindustry. Not only used for fine resolution processing, excimer lasershave been utilized in large-area fabrication and annealing of thin filmsby expanding the beam through optics. The benefit of excimer lasers liesin the surface processing of thin films which is necessary in thefabrication of low-cost, highly efficient solar cells, diodes, etc. Theuse of laser based technique offer several advantages, such as limiteddamage of substrates, high quality films due to heating of a small area,spatial resolution, simultaneous synthesis and patterning, low-costatmospheric pressure processing, fabrication within localized regions,and built-in surface texturing for photovoltaic application.

Methods

With reference to FIG. 2, the current technology provides a method 10for forming a compound on a substrate, which can be used electronicdevices, such as, for example, solar cells (photovoltaics), lightemitting diodes (LEDs), high power devices, devices requires highfrequencies, optical devices, photodetectors, photometers, etc. As shownin block 12, the method 10 includes depositing a composition onto asurface of a substrate. Depositing can be performed by any means knownin the art, such as, for example, spin coating, pouring, brushing, ordipping. The substrate can be any substrate whose surface can withstandhigh temperatures of from about 800° C. to about 1800° C. and thatcontains a first component that is to be incorporated into the compound.Therefore, the substrate is a source of the first component. Thesubstrate can be flat, curved, cylindrical, convex, concave, orotherwise three-dimensionally contoured. Additionally, the substrate canbe crystalline or amorphous, flexible or inflexible, solid, or in powderform. In various embodiments, the substrate comprises silicon,crystalline silicon, amorphous silicon, silicon wafer, or other siliconcoated materials. In some embodiments, the substrate is a powder that iscoated onto another material. For example, a substrate powder, such as,for example, Si powder, can be spray coated onto a “sub-substrate”comprising steel, stainless steel, steel flex (foil), or other material.

The composition that is deposited onto a surface of the substrate can beany composition that contains a second component that is to beincorporated into the compound. Therefore, the composition is a sourceof the second component. The composition is deposited as a thin layer offrom about 0.1 μm to about 10 μm, or from about 0.5 μm to about 5 μm. Invarious embodiments, the composition is a carbon source, such as acarbon-based polymer that may undergo pyrolysis. Non-limiting examplesof suitable carbon-based materials include polymers, such as poly(methylacrylate) (PMA), poly(methyl methacrylate) (PMMA), polyimides, directcarbon sources, such as graphene and graphite, liquid materials, such ascarbon rich oils and carbon based gases, and combinations thereof. Invarious embodiments, after depositing the composition, the substrate andcomposition may be baked for from about 10 seconds to about 5 minutes atfrom about 100° C. to about 250° C. During the method 10, the firstcomponent and the second component react to form the compound. In oneembodiment, the composition comprises a liquid material, such as organicoils, into which the substrate is submerged. Here, a continuous carbonsource is provided during multiple pulse illuminations as providedbelow.

As shown in box 14, the method 10 also includes illuminating thecomposition and the substrate with pulsed energy emitted from a pulsedenergy/heat source. The pulsed energy source can be any pulsed energysource known in the art that focuses heat on the composition andsubstrate by radiating waves or particles. In some embodiments, thepulsed energy source generates from about 10 pJ to about 1000 mJ, orfrom about 1 mJ to about 1000 mJ, or from about 10 mJ to about 500 mJ ofenergy, pulses of from about 10 ns to about 1000 ns, from about 10 ns toabout 100 ns, or from about 20 ns to about 30 ns, and a repetition rateof about 100 Hz, about 50 Hz, about 20 Hz, about 10 Hz, or less thanabout 10 Hz. In one embodiment, the pulsed energy source generates fromabout 200 mJ to about 1000 mJ, with 25 ns pulses, and a repetition rateof from about 1 Hz to about 100 Hz. Non-limiting examples of pulsedenergy sources include excimer lasers, plasma arc melting (PAM), andpulsed particle beams. Excimer lasers include lasers comprising Ar₂ (126nm), Kr₂ (146 nm), F₂ (157 nm), Xe₂ (172 and 175 nm), ArF (193 nm), KrF(248 nm), XeBr (282 nm), XeCl (308 nm), XeF (351 nm), KrCl (222 nm).Pulsed particle beams include pulsed linear particle accelerators(linac's), such as pulsed linac ion accelerators, pulsed linac protonaccelerators, pulsed linac electron accelerators, and electron-beamsintering machines.

In various aspects, illuminating the composition and the substratecomprises moving the pulsed energy source relative to the substrate andcomposition or moving the substrate and composition relative to thepulsed energy source, such that the compound will only be formed inregions that are directly exposed to the pulsed energy. Accordingly, asshown in boxes 16 and 18 the method 10 also respectively includesmelting the substrate and decomposing the composition. Because meltingand decomposing result from exposure to the pulsed energy source, theymay occur simultaneously or substantially simultaneously. Moreparticularly, when the substrate surface is illuminated with the pulsedenergy source, it undergoes melting and re-crystallizes as it cools inbetween the pulses with a repetition rate of less than about 100 Hz orof about 1 Hz. Therefore, the melting briefly frees the first componentfrom the substrate as a “melt.” As the substrate is melting, thecomposition is decomposing, for example, by pyrolysis. Therefore,decomposing briefly frees the second component from the composition. Asshown in box 20, the method then comprises forming a new compound on thesubstrate. Specifically, the second component diffuses into the meltcomprising the first component, and upon cooling between pulses, thecompound is formed on the substrate. Because the compound is only formedin regions of the substrate and composition that are exposed to energypulses, surfaces of the substrate and composition that are not exposedto energy pulses are not exposed to high temperatures, i.e., theysubstantially remain at room temperature. This selective exposure topulsed energy permits the compound to be deposited in any pattern, suchthat selective growth of a compound on a substrate is achieved. Forexample, the compound can be deposited as a film, a layer, pattern, orinterconnected patterns or layers. Upon completion of the method 10,additional electronically active layers or contacts may be deposited onthe compound or opposing surface of the substrate. The additional layersmay be deposited by any method known in the art, including, for example,by vapor deposition, plasma enhanced chemical vapor deposition,epitaxial growth, e-beam evaporation, dipping, and solution coating.

Single side heterojunction devices or double side heterojunction devicescan be generated by the method 10. For example, after the method 10 hasbeen completed on a first side of a substrate, the method 10 may berepeated on a second opposing side of the substrate to form a doubleside heterojunction. FIG. 3A shows a schematic of an exemplary solarcell device having a double side heterojunction and FIG. 3B shows asimplified energy band diagram of the device shown in FIG. 3A. Here, inplace of the a-Si:H buffer layers depicted in the device shown in FIG.1, β-Sic layers are used on both sides of the substrate. These layersare formed by driving carbon into Si melt at the surface of a c-Sisubstrate by the method 10 of FIG. 2.

As discussed above, the method 10 can be used to form high qualityheterojunction devices. Therefore, a heterojunction device is alsoprovided. The heterojunction device a substrate and a compound depositeddirectly on the substrate, wherein the device is made by depositing acomposition onto a surface of a substrate; illuminating the compositionand the substrate with pulsed energy emitted from a pulsed energysource; melting the substrate and decomposing the compositionsimultaneously; and depositing the compound on the substrate, wherein afirst component of the compound is derived from the substrate and asecond component of the compound is derived from the composition. Themethod for making the device and the types of substrates, compositions,and compounds that characterize the device, may include any combinationof limitations provided herein.

Method Variations

An exemplary embodiment of the current technology is shown in FIG. 4A,where a single side heterojunction solar cell is fabricated on a c-Si(p-type, N_(A)˜2.5×10¹⁴/cm³) substrate using excimer laser pulses(energy=300 mJ, pulse=25 ns). FIG. 4B shows an optical micrograph of theexemplary device with a resulting compound comprising silicon carbide(SiC). As shown in the figure, a thin silicon wafer (250 μm) (FIG.4A(i)) is coated with thin layer (1 μm) of PMMA (FIG. 4A(ii)). Then, theSi and PMMA are exposed to laser pulses (FIG. 4A(iii)). A carbon layeris formed by pyrolysis and at the same time the surface of Si is meltedby the high power laser pulse. Carbon diffuses into the melt during thistime and is immediately frozen in the melt upon cooling. A SiC film isformed during the cooling. An Al contact is shown on an opposing surfaceof the silicon wafer.

FIG. 5 shows a simplified band diagram of the heterojunction shown inFIG. 4B. A photomicrograph of an exemplary heterojunction generated bythe current methods is shown in the inset of FIG. 6A. The exemplaryheterojunction shown in FIG. 6A was generated by illuminating a polymerdeposited on a silicon substrate with an excimer laser. Accordingly, theexemplary heterojunction includes a Si substrate with a layer of SiC.FIG. 6A also shows Raman spectra of the SiC on the Si substrate over awide wavelength range. FIG. 6B shows an exploded view of the peak ofFIG. 6A located immediately prior to 1000 cm⁻¹. Raman spectra werecollected using a 532 nm laser source under ambient conditions (ambienttemperature and ambient pressure). The strong intensity at 520 cm⁻¹shown in FIG. 6A originates from the Si substrate. Spectra between 930cm⁻¹ and 990 cm⁻¹ shown in FIGS. 6A and 6B are due to acoustical andoptical phonon modes of cubic or hexagonal polytypes SiC. Peakbroadening is related to the damping of phonon modes due to short rangeordering of SiC crystallites and the effects of surroundings having Si,as well as C-clusters. The change in number of SiC bond by irradiationby different number of pulses could be inferred from a change in theintensity of Raman band. Such a high quality SiC film on Si substratecan be used in the fabrication of SiC-Si heterojunction solar cells,replacing CVD grown a-Si:H layer.

FIG. 7 shows measured J-V results over a large voltage range for devicesproduced by varying the number of laser pulses. Devices with very highbreakdown voltages, such as breakdown voltages greater than about 200 V,can be fabricated using this approach. For these devices, contacts aredeposited by any method known in the art. For example, an aluminum (Al)back contact may e-beam deposited or generated by using aluminum paste.The J-V measurements of FIG. 7 show that high quality Si-SiCheterojunction devices can be formed with optimum number of pulses(optimum power transfer). The devices were measured over a large voltagerange using a Keithley 2400 SourceMeter® source measurement unit(Keithley, Cleveland, Ohio). For the best performing diode (2 pulses) arectification ratio of 3×10⁴ (at ±1V) was obtained. It also shows a highreverse breakdown voltage of >200V (source limited) with leakage currentof 6 μA/cm² and 44 μA/cm² at reverse bias of 1 V and 50 V, respectively.This leakage current is much smaller than previously reported for SiC/SiHJD's. The breakdown voltage for various devices is much higher than 200V, but results shown here are limited by the measurement setup.

The leakage current increases for devices made using 4 and 8 pulses,which can be contributed to the edge effects, which is more prominent atsharp edges. Diodes may break down in regions where a breakdown field isreached first. These preliminary J-V measurements show that high qualitySi-SiC HJDs can be formed by selecting optimum number of pulses (optimumpower transfer). The SiC/Si material system experiences ˜20% lattice and8% of thermal expansion coefficient mismatch, and thus the CVD grown SiCcontains a large number of extended defects, which results in a highleakage currents in 3C-SiC/Si devices. Here, the low leakage currentindicates fewer defects at the interface. This is achieved due to bandgrading due to variation of Si to C ratio across the junction. Thedevices fabricated using 1 and 2 pulses show the highest reversebreakdown voltage with very small leakage current densities. The higherleakage current realized for 4 and 8 pulses can be attributed to carbonrich sharper edges. Thus, such diodes will break down in these regionsfirst due to high field strength.

These preliminary J-V measurements show that high quality Si-SiC HJDscan be formed by selecting optimum number of pulses. The SiC/Si materialsystem experiences ˜20% lattice and 8% of thermal expansion coefficientmismatch, and thus the CVD grown SiC contains a large number of extendeddefects which results in high leakage currents in 3C-SiC/Si devices. Thelow leakage current in our fabricated devices shows fewer interfacedefects. From J-V measurements it can also be concluded that theconductivity of SiC layer is n-type which is due to unintentional dopingof SiC with shallow donor nitrogen, which has a low binding energy of15-20 meV. In the fabrication of SiC layer using a laser, nitrogen fromthe ambient environment is incorporated in the film during diffusion ofcarbon into Si melt.

FIG. 8 provides a graph showing measured capacitance-Voltagecharacteristics. n type SiC/p-Si HJD'S were measured at 100 kHz withdifferent number of pulses.

In anisotype heterojunctions, like n-SiC/p-Si, the capacitance as afunction of applied bias voltage is given by the relation in equation 1,where N_(d) is the effective density of state of n-type β-Sic (donor),N_(a) is the density of acceptor impurities of p-Si, ∈_(d), and ∈_(a)are the dielectric constants of SiC and Si respectively, ∈₀ is thepermittivity of free space, V_(bi) is the built in potential, V is theapplied voltage, C is the capacitance and A is the area of HJ.

$\begin{matrix}{\frac{C}{A} = \sqrt{\frac{{q\varepsilon}_{0}\varepsilon_{\alpha}\varepsilon_{d}N_{\alpha}N_{d}}{2\left( {{N_{\alpha}\varepsilon_{\alpha}} + {\varepsilon_{d_{\alpha}}N_{d}}} \right)\left( {V_{bi} - V} \right)}}} & {{equation}\mspace{14mu} 1}\end{matrix}$

Given the density of acceptor impurities (p-Si) about 2.0×10¹⁴ cm³ andassuming the dielectric constant of SiC to be 11, the doping level inβ-Sic is about 5×10¹⁵ cm⁻³.

FIG. 9A is a graph that shows measured J-V characteristics withdiffering number of pulses and J-V characteristics of a device withsingle pulse under dark and illuminated conditions is shown in FIG. 9B.The device has a current density (Jsc) of 17 mA/cm², open circuitvoltage (Voc) of 0.33 V and fill-factor (FF) of 62%. The devices clearlyshow good optical conversion efficiency (˜8%). Previously, similarresults were obtained using CVD grown and sputter deposited SiC/Si HJ.The measured internal quantum efficiency (IQE) spectra for a SiC/Si HJDbased solar cell is also shown in FIG. 9C. The spectrally resolved IQEexhibits a peak at about 650 nm, which coincides with a peak inpreviously reported SiC/Si solar cells fabricated using CVD andsputtering processes. The graph shows a good optical conversionefficiency of about 8%. The FF can be improved by depositing a thinconductive layer on SiC. Further improvement can be achieved by formingthe devices on both sides of the substrate. This process is simpler thancommon chemical vapor deposition (CVD) processes and requires fewerfabrication steps, and more importantly, it does not require vacuumprocessing, which is an expensive fabrication step. These results showthat high quality SiC layers are formed on Si substrates using laserprocessing.

Band-Gap Tailoring or Engineering.

The current technology also provides for band-gap tailoring of compoundsdeposited on substrates. A spatial variation of the band-gap within theabsorbing region of a solar cell can be optimized to enhance absorptionand improve probability of carrier collection. This optimization leadsto increases in short circuit current density (Jsc) and open circuitvoltage (Voc), which in turn help to improve solar cell efficiency. Bandgap tailoring allows in the design of optimum field profile to enhancecollection efficiency. Here, the band-gap of a compound deposited on asubstrate can be tailored by varying the first component: secondcomponent ratio in the compound. For example, the band-gap is madelarger by increasing the concentration of the second component in thecompound. In various embodiments, the first component:second componentratio is about 1:1 to about 1:0. In some cases, to form interconnectsthe ratio may be about 0:1. More particularly, the firstcomponent:second component ratio can be varied by adjusting the energypulse duration, amplitude power, and/or repetition rate. Accordingly, asshown in block 20, in various embodiments the current method 10 of FIG.2 optionally includes tailoring a first component:second component ratioin the compound by adjusting at least one of the energy pulse duration,amplitude power, or repetition rate to create a thermal profile in thesubstrate that naturally controls the diffusion of the second componentinto the melt comprising the first component.

As an example of film grading, a device having a Si substrate and acomposition comprising a carbon source can be combined to form a film ofsilicon carbide (SiC). The bandgap of the SiC film can be tailored byvarying the Si to C concentration in the SiC. The band gap can be madelarger by increasing the carbon concentration (limited to a maximum of1:1 ratio) in the film. The bandgap can be varied between Eg ofc-silicon (2.38 eV) to Eg of SiC (3.26 eV). The Si:C ratio can be variedusing, for example, laser pulses. The pulse energy, power density, andrepetition rate can be changed to create a thermal profile in thesubstrate that naturally controls the diffusion of carbon into siliconmelt. The device depicted in FIG. 3A has a graded SiC layer as can becharacterized by the smooth transitions shown in the energy band diagramof FIG. 3B. A ratio gradient can also be designed to achieve low stressfilms of SiC on Si.

As discussed above, high quality heterojunction devices with gradedband-gaps can be prepared by the current method. Accordingly, thepresent technology also provides a heterojunction device comprising asubstrate comprising a first component and a compound layer comprising asecond compound and the first component deposited directly on thesubstrate and formed by the methods described herein, wherein thecompound layer comprises a concentration gradient. The concentrationgradient is characterized by an increasing or decreasing firstcomponent:second component ratio from a compound surface in contact withthe substrate to an opposing compound that is not in contact with thesubstrate. In various embodiments, the heterojunction device comprises aSi substrate and a compound layer comprising SiC with a Si:C ratio. TheSi:C ratio increases or decreases from a SiC surface in contact with theSi substrate to an opposing SiC surface that is not in contact with theSi substrate.

Surface Texturing.

Antireflection (AR) layers or coatings are often applied to solar cellsto improve their efficiency. An alternative to treating a surface withan AR coating/layer, which is built of many stacks of dielectric films,is texturing of the surface. Textures are typically formed on top offabricated solar cells through elaborate processes, such as wet etching,polymer embossing, etc. Texturing is beneficial as it provides betterperformance than the thin dielectric based AR coating techniques.Moreover, texturing allows efficient coupling and light trapping over awide spectral band. As shown in block 22, in various embodiments themethod 10 of FIG. 2 optionally includes texturing the compound. In thecurrent method 10, texturing is optionally achieved during formation ofa compound layer on a substrate. In particular, texturing is engineeredby adjusting pulse rate and the focal point of the pulsed energy source.The texturing increases with increasing in number of pulses hitting thesurface and thus an optimized number pulses can be used to achievetexturing. In various embodiments, texturing results in a pyramidaland/or Gaussian (sinusoidal like) surface. Without being bound bytheory, the top surface roughness of a solidified melt can be affectedby a rippling effect that occurs due to surface tension forces exertinga shear force on a liquid surface. This affect is primarily due to asurface temperature difference between a source of energy and asolidifying zone caused by the motion of the pulsed energy source. Beamshaping and spot overlapping can also be used for larger scale surfacetexturing.

As an example of texturing, a device having a Si substrate and acomposition comprising a carbon source can be combined to form a film ofsilicon carbide (SiC) with a textured surface. The texturing is achievedduring the formation of SiC layer on Si substrate by controlling thepulse rate and focal point of an excimer laser. FIGS. 10A and 10B showphotomicrographs of textured surfaces formed during SiC growth on apolished Si wafer with an excimer laser.

Carbon Interconnects.

In a typical solar cell, the surface is covered with conductive oxide(such as indium tin oxide, ITO) and silver (or other conductors) byscreen printing to help carry out the current to the battery (amultistep process). Here, the current method is used to print carbonbased conductive traces. Accordingly, the current technology alsoprovides a process for forming interconnects for solar cells or otherdevices by achieving carbonization of a polymer using a pulsed energysource. In particular, interconnects are formed by illuminating asubstrate and composition with a pulsed energy source, wherein energy isfocused such that its power on its periphery is sufficient to carry outpyrolysis of the composition, but low enough that the substrate is notmelted, and its power in a center region is sufficiently high so that itmelts the surface of the substrate to thereby form a compound. Forexample, carbon contacts can be generated by a device by depositing acarbon source on a silicon substrate, and illuminating the substrate andcarbon source with an excimer laser. As shown in FIG. 11A, hexagonalshaped conductive rings are formed on the edges of Si-SiC devices, whichhelps carry current while minimizing the dead space, i.e., the regionshadowed by the interconnects. Through this method, screen printing ofsilver paste can be avoided, which saves labor time and cost ofmaterials. Conductive rings can be formed simultaneously during thegrowth of SiC films. This formation is achieved by controlling the beamprofile using a shadow mask. FIG. 11B shows a fabricated SiC-Siheterojunction device with carbon ring on the perimeter for currentcollection. The excimer laser was focused as such that the power on theperiphery was sufficient to carry out pyrolysis of polymer, but lowenough that Si is not melted. However, the power in the center of thebeam was significantly large so that it melted the surface of the Siwafer and helped form the SiC film.

SiC Doping.

Doping compound films on both sides of a substrate is typically requiredto form efficient double side heterojunction solar cells. A compoundfilm on a first side of the substrate requires an n-type dopant and acompound film on a second opposing side of the substrate requires ap-type dopant. Doping of a compound film of, for example, SiC iscomplicated by two facts: (i) the dopant can occupy either the Si or theC site; and (ii) the diffusion rate in SiC, which has a tight bond anddense structure, is slow. In various embodiments of the currenttechnology, SiC layers may be doped with donor nitrogen, which is showsn-type behavior. The origin of the n-type doping is due to shallow donornitrogen with a binding energy of from about 15 to about 20 meV. Thedonors can be present in a deposited SiC film in concentrations of lessthan about 10¹⁸ cm⁻³. For example, SiC films grown using chemical vapordeposition process are mostly unintentionally n-type doped due topresence of nitrogen source during growth from gas precursors such asmethylsilane (99%), or due to other contaminates. Nitrogen is also themost commonly used n-type dopant for SiC. Traditionally, a nitrogendoped film can be grown by addition of Nitrogen gas to the source gasduring CVD process. The carrier concentration can be controlled bychanging the mole ratio of N₂ gas to other gases (used for SiCsynthesis). In situ doping of sputtered SiC can be achieved byintroducing nitrogen into the electric discharge during the growthprocess. In the fabrication of SiC layer according to the presenttechnology using a pulsed energy source, such as an excimer laser, asilicon substrate and a composition comprising a carbon source, nitrogenfrom the ambient atmosphere is incorporated into the SiC film duringdiffusion of carbon into a Si melt. Therefore, the current methodoptionally includes doping the compound with nitrogen.

Thermal diffusion, ion implantation and spin on doping are the mostcommon methods of doping SiC. Thermal diffusion of dopants requires highprocessing temperatures and can cause impurity contamination anddeterioration of crystallizing SiC. Ion implantation can also severelydamage the lattice structure. Laser-induced doping of SiC films has beenused in past to dope SiC without the use of very high temperature. Here,pulsed energy sources, including excimer lasers, can be used to assistdoping of SiC with nitrogen, aluminum, chromium, phosphorous and boron.Doping with excimer lasers offers the advantage of locally increasingthe temperature without heating the whole substrate. Moreover bothdoping and dopant activation can be achieved in a single process. Thehigh power pulsed laser (excimer laser) with nanosecond durationsenables the application of a large amount of energy in a short duration.Under controlled conditions, surface melting of SiC does not exceed adepth of a few hundred nanometers for the rapid solidification from thebulk, allowing the dopant to be incorporated by liquid phase diffusion.This is same as principle of laser induced dopant incorporation for Siwhich is also a melt/growth process.

P-type doping of SiC can be achieved with a boron spin-on dopantsolution and irradiation with a pulsed energy source, such as a highpower excimer laser pulse. Laser based doping can also be carried outfor p-type doping using spin-on dopants (SOD) as impurity sources.Spin-on doping permits precise control of dopant through application ofa homogeneous solution to the substrate. Laser pulses liberate thedopant atoms from spin coated thin film through thermal dissociationwhere laser heats the sample. Thereafter, the dopant undergoes rapiddiffusion into liquid phase substrate material. The dopant atomsrearrange themselves into the lattice upon cooling.

The advantage of using lasers for doping is that it allows only shallowdepth as the excimer laser is strongly absorbed in the surface region ofthe wafer. Furthermore, laser processes allow doping in selected regionsand also prevents redistribution of impurity profiles due to very shortdurations of pulse irradiation. This method does not require expensiveequipment, ultrahigh vacuum or toxic gas sources. The thickness of thedoped layer depends on the amount of energy absorbed by SiC. SiCexhibits high absorption at 248 nm, which results in thin doped layers.Excimer lasers also provide concentrated localized temperature rises,which also help in diffusion of boron. Due to localized heating, thetemperature decreases quickly, which causes the dopant to become trappedin the film. Therefore, a doping profile may be controlled by adjustingthe energy density of the pulses.

Use of Low Cost Unpolished Wafers.

One of the major contributing expenses to the manufacture of solarcells, diodes, etc. is the silicon substrate. From the growth of theboule to final wafer production, a considerable amount of cutting,grinding, lapping and polishing is performed. Lapping and polishing canamount to as much as 50% of the substrate cost. Thus, in the manufactureof solar cells, as-cut Si wafers (without lapping and polishing) aredesirable. As discussed above, excimer lasers have been used totransform thin layers of amorphous Si (50-200 nm) into high qualitypolycrystalline Si with greatly enhanced electron mobility, for use inflat-panel displays for mobile phones and flat-screen televisions. Ascut, wafers have many surface defects and these defects can beeliminated by laser melting a surface layer. In accordance with thepresent technology, during the formation of compound layers, such ascompound layers of SiC, the surface of the substrate, such as a Sisubstrate, undergoes melting and solidification, which naturally reducessurface defects. Therefore, the substrates used in the current methodscan be unpolished.

FIG. 12 shows a micrograph of a device fabricated on an unpolished(as-cut) 250 μm thick silicon wafer (NA˜10¹⁵/cm³). As shown in thefigure, there is a clear contrast between the SiC region formed from thesilicon melt and the neighboring unpolished Si region. As shown in theexploded view on the right side of the figure, the SiC structure forms atexture that is useful as an anti-reflection layer. J-V characteristicsof this device were also measured and are shown in FIG. 13A under darkand illuminated conditions. Measured J-V of different devices formedusing different number of pulses is also shown in FIG. 13B. In thesedevices, aluminum was coated on the back side on p-type Si, and no metalcoating was carried out on the SiC layer. J-V of the device was measuredby probing the perimeter region, which is carbon rich (carbon ring).These preliminary results show that high quality Si-SiC heterojunctiondevices can be formed on low cost unpolished Si wafers. In particular,the pulsed energy source, such as an excimer laser, melts and improvesthe quality of the surface region while forming a SiC layer.

Thin Si Growth from Small Particles.

In various embodiments, the current method also includes sintering ofsilicon particles spray deposited on a thin metal substrates (foil)prior to the formation of SiC layer. Si powder (micron to submicronsized particles) coated on a metal foil is melted using a pulsed energysource, such as an excimer laser, to achieve high quality Si growth on alow cost large area substrates. This embodiment reduces Si substratecost and enables the growth of solar cells on 3D platforms, such as, forexample, rooftops of a cars, house sidings, etc. Silicon on metal foilis compatible with a roll-to-roll process (similar to newspaperprinting) which is known to be an ultra-low cost process as it can becompletely automated. The metal foil also acts as the back conductor asneeded to form a close circuit loop. The growth of Si and formation of aSiC layer can be carried out in parallel by using two pulsed energysources, such as excimer lasers, working in parallel. For example, thefirst laser sinters and forms a thin Si layer on a metal sub-substrateand the second laser forms the SiC layer on the Si.

Embodiments of the present technology are further illustrated throughthe following non-limiting examples.

Example 1

SiC/Si heterojunctions (HJs) are a promising candidate for a wide rangeof applications such as high-voltage converters, photovoltaics, highpower high frequency devices, high temperature circuits, optical diodes,etc. SiC is an excellent material for these applications due to its widebandgap, high thermal conductivity, high electron mobility, and chemicalstability. Out of the various existing polytypes, β-Sic exhibits thelowest bandgap and excellent electronic properties such as high electronmobility.

Different growth techniques have been investigated to achieveheterojunctions (HJ) with low crystal defects that occur due to thelattice mismatch between the SiC and Si. Chemical vapor deposition (CVD)and magnetron sputtering are the most common techniques used to depositSiC on Si substrates; however, they require high vacuum and hightemperature (>1000° C.) for growth of high quality SiC. The thin filmsgrown at very high temperatures leads to high stress and formation ofcracks due to lattice mismatch and difference in thermal expansionco-coefficients. In addition, HJ's formed at high temperatures can leadto diffusion of dopants from Si to SiC. Therefore, a growth techniquethat does not require high temperature annealing of Si substrate andalso a technique that does not require vacuum processing is desirable toreduce manufacturing cost.

Recently, laser synthesis techniques have been used to transform solidcarbon sources to graphene directly on Si and quartz substrates. Highpower lasers, such as excimer lasers, have also been used to transformthin layers of amorphous Si (50-100 nm) into high qualitypolycrystalline Si through melting and recrystallization. As discussedherein, laser irradiation with very high power is capable ofdissociating solid carbon sources and also melting silicon which cangive rise to a new mechanism of SiC growth.

Here we report a novel method to synthesize SiC using pulsed KrF excimerlaser radiation. This technique is based on focusing a laser beamthrough a carbon (C) film layer onto an absorbing substrate (Si) andcreating a local hot-spot where the Si surface melts and reactionbetween Si and C takes place to form SiC. A thin coating of polymethylmethacrylate (PMMA) on Si is used to provide the carbon throughpyrolysis. The substrate is held under ambient conditions (pressure andtemperature) during this process, and both polished and unpolishedwafers can be used to form high quality devices. Direct synthesis of SiCon Si using lasers under ambient conditions is reported here for thefirst time.

Device Fabrication

Si C/Si HJ devices (HJDs) were fabricated by growing unintentionallyn-type doped SiC on single side polished p-Si (111) substrate (30-50Ω-cm, thickness=250 μm; “silicon wafer”) using a laser synthesistechnique. The silicon wafer was prepared by first removing the nativeoxide using a buffered hydrofluoric acid (HF) solution followed by spincoating of a thin layer of PMMA (˜1 μm). The silicon wafer was thenplaced on a XYZ manipulator at room temperature and atmospheric pressurewith air background. The silicon wafer and PMMA (the “sample”) was thenirradiated with a high power KrF excimer laser beam (λ=248 nm, pulseduration ˜25 ns). The excimer laser beam was directed onto the substratethrough an optical path that homogenizes and shapes an intensity profileto achieve uniform illumination across a desired focal area.Accordingly, an area of the silicon wafer is irradiated and undergoesmelting and re-crystallization during the pulses (<1 Hz repetitionrate). The laser beam simultaneously decomposes PMMA while melting thesurface of Si. The PMMA provides a solid carbon source through pyrolysisfor SiC synthesis.

Several devices were formed by irradiating polished Si with differingnumber of pulses (1, 2, 4 and 8 pulses). As the pulse width is narrow(˜25 ns), cooling takes place immediately following the pulse. Anoptimum number of pulses needed to form a high quality SiC film wasdetermined by measuring optical and electrical characteristics. Theaverage area of the device obtained was approximately 350×520 μm², whichwas dictated by focusing of the laser beam. Larger devices can be formedby restring the laser beam across a substrate. For this study, the sizeof the devices was limited to the average size achieved withoutrastering. Nickel (Ni˜50 nm) was used to form a contact to SiC andAluminum (Al) was blanket deposited on the backside of the p-Si waferusing e-beam deposition. No annealing was carried out after thedeposition of these metal films prior to any measurements.

Conclusion

Here, an alternative low cost technique for synthesizing a n-SiC/p-Siheterojunction under ambient conditions using lasers is demonstrated.The electrical performances shows that devices with very high breakdownvoltages (>200 V) can be fabricated using this approach. Diodes showsgood rectification ratio of 3.0×10⁴ at ±1.0 V and leakage currentdensity of 6 μA//cm2 (1 V). A solar cell with efficiency of 8% isdemonstrated. The device quality may further be improved by reducingseries resistance and further optimizing laser pulse power. This processmay be utilized in the growth of SiC devices through post processing ofcomplementary metal-oxide-semiconductor (CMOS) wafers.

Example 2

This example presents the growth and characterization of SiC/Siheterojunction diodes for microwave circuit applications. A novelprocess based on selective growth of SiC using KrF excimer laser on apolymer coated Si wafer under ambient conditions (in air at atmosphericpressure and at room temperature) is presented. Laser irradiation withhigh power leads to dissociation of solid carbon sources and melting ofsilicon simultaneously, which leads to growth of SiC. A Raman spectrumof the grown samples shows peaks for acoustical and optical phonon modesfor β-Sic between 940 cm-1 and 980 cm-1. The device fabrication processis post-CMOS process compatible as it allows localized growth of SiC onSi and thus offering limited damage to the substrate and surroundingcircuits. The fabricated diodes show high breakdown voltage (>200 V) andlow leakage current density of 3-5 μA/cm2 (−5V). Experimental resultsfor DC characteristics, microwave rectification and frequencymultiplication are presented. The diodes are fabricated on two types ofSi wafers with different doping densities. The diodes fabricated onwafer with higher doping show lower series resistance and thus performbetter at higher frequencies. The best performing diode worksefficiently as microwave rectifier with rectification sensitivity ofabout 8.4 mA/W at 3 GHZ. The diodes also work well as frequency doublerover wide frequency range of 2-6 GHz. This laser fabrication process haspotential in developing low cost, high power microwave diodes byenabling direct localized growth on Si substrates.

Introduction

Affordable high power microwave devices are desired for emergingapplications in wireless communication as well as for continuousprogress in defense applications. High power RF devices haveapplications in base station transceiver systems, high-speedcommunications, automotive collision avoidance and homeland security.High frequency solid state devices are limited by transit time and thusrequire smaller sizes. However, compact devices suffer from increasedtemperature of operation as they cannot handle high power densities.Wide bandgap materials like GaN and SiC are of interest as they canhandle high power densities unlike conventional semiconductors which arelimited by saturated charge carrier velocity at high electric field.Over the past two decades, a variety of microwave devices based on GaNand SiC, including metal semiconductor field effect transistors(MESFETs), heterojunction bipolar transistors (HBTs), high electronmobility transistors (HEMTs), static-induction transistors (SITs), andimpact ionization avalanche transit-time diodes (IMPATT) have beendemonstrated. Recently, SiC has regained interest for powerapplications. In comparison to GaN, SiC has higher thermal conductivityand can theoretically operate at higher power densities. Although SiChas lower carrier mobility, it is still adequate for transistorsdesigned for high-power operation in the microwave frequency range andalso exhibits high saturation velocity. SiC exists in a large number ofpolytopes. In comparison to other polytypes, 3C-SiC has a smallerbandgap, but has higher electron mobility and is thus advantageous indevice applications, such as MOSFETs. Additionally, it is possible togrow 3C-SiC epilayers on Si which makes it a less expensive alternative.Integration of high power materials with Si is very important to enablemass production of low cost, high power devices for commercialmillimeter-wave applications. Integration with Si is also very importantfor future millimeter-wave integrated circuits which require CMOScircuitry for data conditioning and signal processing, and mixed signalcircuits.

The most commonly used integration techniques are hetero-epitaxy ofdifferent semiconductors and hybrid integration approaches (non-epitaxy)techniques. Traditional hybrid integration approaches such as wirebonding, flip chip, multichip assemblies provide short term solutions.Direct growth of compound semiconductors (CS) on silicon substrates isdesired. The biggest challenge for direct growth is lattice mismatch andcoefficient of thermal expansion (CTE) mismatch. Thermal mismatch is abigger problem as the growth of GaN or SiC is usually carried out attemperatures above 1000° C. The use of such high temperatures can alsocause re-distribution of dopants in Si substrates and the accumulationof thermal mismatches at junctions.

In recent years, different techniques have been used to solve thischallenge using approaches such as buffer layer engineering in epitaxyof GaN on Silicon to suppress crack formation and use of low temperaturemolecular beam epitaxy (MBE) or plasma enhanced chemical vapordeposition (PECVD) for depositing SiC. A more attractive approach ascompared to large area epitaxy on Si is localized growth. Localizedsmall area epitaxial growth can potentially help in reducing misfitdislocation density in lattice mismatched systems by reducingdislocation interaction and multiplication. As the growth area islimited to few mm²′ the quality of heterogeneous grown CS on silicon canbe better optimized. This also allows direct/selective integration ofCMOS and high power microwave devices on a common silicon substrate.Recently, local epitaxy of InP is carried out on the lithography definedLattice Engineered Substrate Si wafer (growth windows). However, thesetechniques still require complex epi-layer engineering and use of uniquesilicon substrate wafer for local epitaxial growth. Thus, thesetechniques are not very cost effective. In addition, no local epitaxytechnique has been demonstrated for SiC. So, there is a need foralternate techniques for the growth of SiC on Si for heterogeneousintegration of high power microwave circuits along with CMOS devices.

Other than direct deposition of SiC on Si using traditional techniquessuch as CVD, PECVD and MBE, other indirect techniques have also beendemonstrated. For example, growth of SiC on Si using thermal annealingof pre deposited C60 film which causes diffusion of carbon into Si.Rapid thermal annealing (RTA) has also been used to grow SiC on Sisubstrates. RTA requires much lower temperature than the conventionalCVD or furnace annealing (FA) because of enhanced SiC crystallization athigh heating rates. Although RTA annealing has advantages over FA, itstill requires heating of the whole substrate and thus is not post-CMOSprocess compatible. However, laser based annealing is a promisingtechnique to achieve localized heating. Laser based annealing has beenused as an alternative to classical thermal annealing for variousapplications such as doping of crystalline Si and SiC, recrystallizationof amorphous Si wafer for thin film transistors.

High power excimer lasers have the ability to deliver large amounts ofenergy into a confined region of material in order to achieve desiredtemperature response. Selective growth of SiC/Si heterojunction diodesusing high power KrF (nanosecond) excimer laser annealing has beendemonstrated herein. The technique is based on focusing a laser beamonto a carbon rich polymer film layer directly deposited on a Sisubstrate. Laser irradiation with very high power is capable ofdissociating solid carbon sources and melting silicon, which leads togrowth of SiC. The key mechanism for laser processing on semiconductorswith a high power laser is photo-thermal (pyrolytic) where the absorbedlaser energy is directly transformed into heat. High power laser withfluences above the threshold of melting of material can lead to muchhigher solubility than in the solid phase, resulting in rapid materialhomogenization. In addition, laser based annealing offers many fasterdiffusion paths for C atoms to diffuse into Si and form SiC formation.Also, due to extremely high heating and cooling rates in significantchanges can occur to the material in short durations. For instance,enhanced diffusion rates for impurity doping, and the reorganization ofthe crystal structure.

Most semiconductor materials have a strong absorption in the UV spectralregion and the energy is absorbed near the surface region modifyingsurface chemistry, without altering the bulk. Therefore, this processoffers limited damage to the substrate and neighboring circuits. Theother advantages of using laser based technique for growth of SiC aregrowth under ambient conditions, rapid heating and cooling rate, andpost CMOS compatibility. The details of the laser based process, and DCand RF characterization of SiC/Si heterojunction diodes are presented inthis paper. Fabricated diodes show high breakdown voltage (>200 V), highrectification ratio and low leakage current densities. The diode alsoworks efficiently as high power microwave rectifier and frequencydoubler.

Device Fabrication

SiC/Si diodes were fabricated on two different types of wafers, both arep-type doped having different carrier concentrations and thickness. Thediodes made on low doped wafer carrier concentration of 4.3×1014 cm-3,thickness=250 μm, are refereed as diode type A and a diode made on awafer with a higher doping concentration of about 3×1015 cm⁻³ andthickness of 150 μm is refereed as diode type B.

The wafers were first cleaned with acetone and isopropyl alcohol in anultrasonic bath for 5 min. each, followed by a 5 min. rinse in deionized(DI) water. The wafers were then cleaned by etching with a bufferedhydrofluoric acid (HF) solution to remove native oxide followed by a DIwater rinse. This step was followed by spin coating of a thin layer ofPMMA (about 400 nm), and baking for 90 sec. at 180° C. The sample wasthen irradiated with high power KrF excimer laser (λ=248 nm, pulseduration ˜25 ns), FIG. 14. Use of lasers with such smaller wavelengthsallows for local modification of surface properties without altering theproperties of the bulk region. The growth of SiC on Si is carried out bylocalized heating while holding the substrate under ambient conditions(in air at atmospheric pressure and at room temperature). Confinement(localization) of energy is achieved by controlling the laser spatialprofile by focusing a beam through optics, and beam shaping throughhomogenizers, apertures and refractive elements. The irradiated area ofSi undergoes melting and re-crystallization in a very short period oftime. The laser beam simultaneously decomposes PMMA and melts the Sisurface. The PMMA provides a solid carbon source for SiC synthesis. Theaverage area of an as grown device is 350×500 μm² which is dictated bythe focusing optics. To realize RF circuits with working frequencies inthe GHz range, much smaller devices than this area are needed. Thus,further processing is carried out to reduce the device area and todeposit the contact layers. The steps to fabricate small area diodes andto make contacts to Si and SiC are shown in FIG. 14, panels III-VI. TheSiC/Si diodes are also coupled into coplanar waveguide (CPW) feednetwork structures for on-wafer probing and high frequencycharacterization.

First, a thin layer of Ni (200 nm) is deposited by e-beam depositionfollowed by deposition of a 200 nm layer of Al (FIG. 14, panel II). Thebottom Ni is used as an Ohmic contact to SiC while Al is used here as ahard mask for etching SiC using SF₆/O₂ based reactive ion etching (RIE).The metals were patterned and etched to open a window for etching SiC.Al was etched using H₃PO₄:HAc:HNO₃:H₂O (16:1:1:2) and Ni was etchedusing FeCl₃. After patterning of the first layer, SiC was etched usingSF₆ and O₂ plasma, FIG. 14, panel III, using a power of 200 W, a plasmatime of 3 min., SF₆/O₂ flow of 20/5 sccm, respectively. The diodejunction area achieved after etching SiC was about 70×70 μm². Afteretching of SiC, 300 nm of SiO₂ was deposited using PECVD process, FIG.14, panel IV. The oxide was grown to passivate the surface after etchingSiC. The oxide was patterned and etched to open a window on the Si, FIG.14, panel V. After etching SiO₂ using buffer oxide etch (BOE), thesecond metal layer (Al) is deposited which acts as an Ohmic contact tothe Si side of the diode. In a final step, the second metal layer ispatterned to release the diode structures, FIG. 14, panel VI. The Ohmiccontacts on both SiC and Si sides are annealed at 450° C. for 5 min.

The fabricated CPW structure is shown in FIG. 15. The ground (G) pad ofCPW directly contacts the Si substrate, and the signal (S) pad contactsthe SiC layer. The SiC/Si diode characteristics are measured between theG and S pads. Several SiC/Si devices were formed using different numberof laser pulses. The best devices were formed by 2 pulses, similar tothe work presented above. Here, the DC and RF characterization ofdevices fabricated from SiC formed using 2 laser pulses is presented.

Raman spectra for a device fabricated using 2 laser pulses is shown inFIG. 16. The Raman measurement was carried out in backscatteringgeometry using a 532 nm laser. The peak at 521 cm⁻¹ is from the siliconsubstrate and the peaks between 940 cm⁻¹ and 980 cm⁻¹ are due toacoustical and optical phonon modes of β-Sic. Peak broadening is due todamping of phonon modes by short range ordering of SiC crystallites. Theusual forbidden Transverse optic (TO) mode around 796 cm⁻¹ is notobserved here; however, the LO mode peaks matched well with previousresults. The absence of TO mode also suggests the absence of stackingfaults, stress and dislocations at the interface. The inset of FIG. 16shows an optical image of a SiC/Si diode fabricated using 2 laserpulses.

Experimental Results

A. Current—Voltage Characteristics.

All of the measurements presented in this example were carried at roomtemperature and under dark conditions using a Keithley 2400 sourcemeter. The measurements are shown for diodes made with 2 laser pulses(optimum power transfer). FIGS. 17 and 18 show the measured currentdensity-voltage (J-V) characteristics of s large area (500×350 μm²)SiC/Si diode over a large voltage range for diode type I and diode typeII, respectively. Both diodes show a high reverse breakdown voltage ofgreater than 200V with very small leakage current. The breakdown voltagefor the diodes made with 2 pulses is much higher than 200 V, but limitedby measurement set-up. Diode type I shows leakage current density ofabout 3 μA/cm² (−5 V), while diode type II shows slightly higher currentdensity of about 5 μA/cm² (−5V) as expected due to the higher doping ofSi. The measured leakage current is lower than previously reported forSiC/Si diodes fabricated using standard techniques, such as CVD andsputtering. The CVD grown SiC contains a large number of extendeddefects due to lattice and CTE mismatch, which results in high leakagecurrents. Thus, the small leakage current indicates fewer defects at theinterface. This phenomenon can be contributed to localized growth ofSiC, which can potentially help in reducing misfit dislocation. It canalso be concluded from the J-V measurements that the conductivity of theSiC layer is n-type, which is due to unintentional doping of SiC withnitrogen, which has very low binding energy of 15-20 meV. It isconcluded that nitrogen from the ambient is incorporated in the filmduring the laser process. The quality of diode can be accessed byfitting the measurement current-voltage characteristics to the diodeequation, Eq. (2):

$\begin{matrix}{I = {I_{0}\left\lbrack {{\exp \left( \frac{e\left( {V - {IRs}} \right)}{nkT} \right)} - 1} \right\rbrack}} & (2)\end{matrix}$

where, I is the measured current, e is the charge of an electron, Rs isthe effective series resistance/contact resistance, T is thetemperature, k is the Boltzmann constant, I₀ is the saturation current,n is the ideality factor and V is the applied bias voltage. The inset ofFIGS. 17 and 18 show plots of log I vs. V and curve fitted diodeequation at room temperature for diodes type I and II, respectively. Thediode type I shows n=3.4 and Rs=25 kΩ and diode type II shows n=2.8 andRs=6 kΩ. The type II diodes show lower series resistance and higherforward current as expected due to higher doping levels of wafer.

B. SiC/Si Diode Based Microwave Detection

Microwave or millimeter wave detectors are fundamental building blocksfor applications such as wireless power transmission, concealed weapondetection, spectroscopy and medical imaging and energy recycling. Forhigh power rectification devices based on wide-bandgap semiconductorsare required. In this example, performance of SiC/Si heterojunctiondiodes fabricated using laser process was investigated for microwavedetection. All measurements were carried out at room temperature byprobing the devices using a high frequency 50Ω coplanar GSG probe. Forthe measurement, signal (RF+DC) is applied to the device through a CPWprobe, T-Bias and via a directional coupler (HP87300B). The directionalcoupler is used to acquire incident and reflected waves from the device.The reflected/incident signal is measured using a spectrum analyzer. TheI-V characteristics with RF on and off are measured and change betweenthe current at a certain bias point is extracted from the measurements.High frequency losses in the setup are also measured in order toestimate the actual power delivered to the device.

FIGS. 19 and 20 show rectified current as a function of frequency fordiodes type A and B, respectively, and at a fixed bias of about 0.35 V(strongest non-linearity point) and RF power of about 4 dBm. The resultsshow that the rectified current decrease as a function of frequency dueto the parasitics associated with the diode and due to higher impedancemismatch at higher frequencies. The insets of FIGS. 19 and 20 show themeasured rectified current as a function of applied DC bias for diodetype A (1 and 2 GHz) and diode type B (5 and 6 GHz), respectively, andfixed incident power of about 4 dBm. For both diodes, the highestmeasured rectified voltage is near a bias voltage of about 0.35V, whichis close to the strongest non-linearity point of the diode. Diodes typeB performed better than diode type A, producing much higher rectifiedcurrent as expected. For example, diodes type B show rectified currentof 35 μA at 2 GHz while diodes type A show current of 0.86 μA. For diodeB the current remains above about 1 μA over the entire measuredfrequency range (2-7 GHz).

The measured results show that the SiC/Si based diode has a highersensitivity (detection). The higher frequency performance of the deviceis limited by transit time, which is related to the size of the diodeand can be further improved by reducing the area of diode. The maximumRF power applied to the diode is limited by source, but it is expectedthat higher detected current can be achieved at higher power levelwithout breaking down.

FIG. 21 shows measured detected current as a function of input power fordiode type II at 3, 5 and 6 GHz with a fixed bias of 0.35 V. From theslope of FIG. 21, it can be verified that the detected voltage responsechanges linearly with input power over a wide power range (˜10 dBm to 4dBm) for all the measured frequencies and follows the square lawdetection with slight deviation. The measured result shows that thedevice has a sensitivity of about 8.4 mA/W for 3 GHZ signal in themeasured power range. These preliminary results clearly show that theSiC/Si diodes can be used in the design of microwave circuits. The diodeperformance can be further improved by lowering series resistance andcapacitance. The lower series resistance can be achieved by annealingthe contacts in order to reduce the contact resistance, while diodeswith smaller area can be used to achieve lower capacitance.

C. SiC/Si Diode Based Frequency Doubler

Frequency multipliers are often used in a variety of applications, suchas with frequency synthesizers, transceivers and down converters, and infuture applications, such as with 60 GHz broadband wireless systems and77 GHz automotive radar. Development of such multipliers requires use ofhigh power, wide bandgap devices. Any nonlinear component, such asdiodes, varactors or transistors can be used to generate harmonics.Frequency multiplier based on different semiconductor technologies suchas GaAs metamorphic HEMT (mHEMT), SiGe BiCMOS and AlGaN/GaN HEMTs havebeen demonstrated previously. GaN on SiC based HEMTs has led to thehighest power levels achieved so far. There is great interest indeveloping low cost frequency multipliers for commercial MMICtransceivers. Si-based technologies low-cost, high volumecommercialization for compact, single-chip transceivers. However, forpower applications, the mostly hybrid GaN-based multipliers are stillprominent. Here, frequency multipliers based on SiC/Si diodes areprovided. For the experimental set-up, the RF signal from a signalgenerator was supplied to the diode through a directional coupler(HP87300B) and output power at harmonics was measured using a spectrumanalyzer.

Frequency multiplication was measured for diodes with no externalimpedance matching circuits. Second order frequency multiplication wasobserved over a wide frequency range. The frequency multiplicationresults are only presented for Type II diodes as Type I diodes have verylow output power at a second harmonic due to a higher series resistance.FIG. 22 shows the output power of the second harmonic for a type IIdiode at fundamental frequencies in the range of 2-6.5 GHz at inputpower level of −3 dBm. The diode shows an output power −53 dBm for2×fin=4 GHz, and the highest measured output power remains above −72 dBmfor frequencies up to 2×fin=12 GHz. Higher input power levels are notused here due to presence of source harmonics. Considering the largesize of the device (70×70 μm²), the device provides good performance inthe GHz frequency range. The conversion efficiency of the diodedecreases at higher frequencies due to transit time loss and impedancemismatch. Device size can be decreased to enhance performance at higherfrequencies. FIG. 23 shows the output power of the second harmonic as afunction of input power at fundamental frequencies of 2 and 4 GHz. Theoutput power increases linearly with input power over the entire inputpower range, which demonstrates stable operation of the diode. Also, toachieve higher order harmonics, a power source with lower noise levelscan be used to avoid source harmonics or a low pass filter can be usedin the measurement set-up. The device should show a linear response ateven high power levels as clear from results shown by rectificationwhere high power levels were successfully applied without breakdown ofthe diode. The diodes were measured multiple times (more than 10 times)to make sure they can withstand cycles of high power levels and the sameresults were achieved every time. Employing impedance matchingtechniques and by using bandpass filters to filter higher frequencyharmonic content from the source can improve the results. Higherefficiencies can be achieved by supplying more RF input power to thedevice.

Conclusion

SiC/Si heterojunction diodes are fabricated and tested for RF andmicrowave circuit applications. SiC is grown directly on a Si substrateusing an excimer laser process. This process is performed at a low cost,is simple to implement, and can potentially serve as an alternative toconventional techniques for growing SiC on Si locally for heterogeneousintegration of high power microwave circuits directly integrated on aCMOS wafer. The small area diodes for RF characterization weresuccessfully fabricated, showing non-linear I-V characteristics with asmall leakage current. Diodes fabricated using a high doped wafer showhigher forward current and provide better RF characteristics due tolower series resistance. The best performing diode shows a microwaverectification sensitivity of 8.4 mA/watt (at 3 GHz). The envisionedapplications of this rectifier are in high power wireless powertransfer. In addition, SiC/Si heterojunction diodes were tested asfrequency doublers. Diodes on higher doped wafers show frequencymultiplication in the frequency range of from about 4 to about 12 GHz.High frequency high power diodes are demonstrated using a new lasergrowth technique which can serve as a basic building block for highperformance microwave circuits, such as transceivers. Diodes with evensmaller areas made using the technique can operate at millimeter wavefrequencies and higher. These results clearly attest to the possibilityof high-performance SiC-based electronic devices on low-cost, large areaSi substrates. This technology can potentially reduce cost, and improveperformance for applications associated with high-speed communications,automotive collision avoidance and homeland security weapons detection.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method for forming a compound on a substrate,the method comprising: depositing a composition onto a surface of asubstrate; illuminating the composition and the substrate with pulsedenergy; melting the substrate and decomposing the compositionsimultaneously; and forming a compound on the substrate, wherein a firstcomponent of the compound is derived from the substrate and a secondcomponent of the compound is derived from the composition.
 2. The methodaccording to claim 1, wherein the illuminating the composition and thesubstrate comprises illuminating the composition and the substrate withpulsed energy emitted from an excimer laser, plasma arc melting, or apulsed energy beam.
 3. The method according to claim 2, wherein thepulsed energy is emitted form an excimer laser comprising Ar₂, Kr₂, F₂,Xe₂, ArF, KrF, XeBr, XeF, or KrCl.
 4. The method according to claim 2,wherein the pulsed energy is emitted from a pulsed linear particleaccelerator (linac) selected from the group consisting of pulsed linacion accelerators, pulsed linac proton accelerators, and pulsed linacelectron accelerators
 5. The method according to claim 1, wherein thedepositing a composition onto a surface of a substrate comprisingdepositing a composition comprising a carbon-based polymer on thesurface of a Si substrate.
 6. The method according to claim 5, whereinSi is the first component and C is the second component, and the forminga compound on the substrate comprises forming SiC on the Si substrate.7. The method according to claim 1, wherein the compound and thesubstrate comprise a heterojunction for an optical device or aphotovoltaic device.
 8. The method according to claim 1, wherein thedepositing a composition onto a surface of a substrate comprisesdepositing a composition onto a surface of a silicon substrate.
 9. Themethod according to claim 3, wherein the silicon substrate comprisesamorphous silicon or crystalline silicon, in solid or powder form. 10.The method according to claim 1, wherein the substrate comprises anunpolished silicon wafer.
 11. The method according to claim 1, whereinthe depositing a composition onto a surface of a substrate comprisesdepositing a composition selected from the group consisting ofpoly(methyl acrylate), poly(methyl methacrylate) (PMMA), polyimides,graphene, graphite, and combinations thereof onto the surface of thesubstrate.
 12. The method according to claim 1, wherein the compositioncomprises a liquid material and the depositing comprises submerging asilicon substrate into the liquid material to thereby provide acontinuous carbon source during multiple pulse illuminations.
 13. Themethod according to claim 1, wherein the illuminating the compositionand the substrate comprises illuminating the composition and thesubstrate with pulsed energy of from about 200 mJ to about 1000 mJ, apulse duration of from about 10 to about 50 ns, and a pulse repetitionrate of from about 1 Hz to about 100 Hz.
 14. The method according toclaim 1, wherein the depositing, illuminating, melting, and forming areperformed at ambient pressure and temperature.
 15. The method accordingto claim 1, further comprising tailoring the band-gap of the compound byincreasing the concentration of the second component in the compositionsuch the first component and second component are present in a firstcomponent:second component ratio of from about 1:1 to about 1:0 andadjusting the pulse duration, amplitude power, and/or repetition rate ofthe pulsed energy.
 16. A method for forming SiC on a silicon substrate,the method comprising: depositing a carbon source on a surface of a Sisubstrate; illuminating the carbon source and Si with an excimer laserthat generates from about 1 mJ to about 1000 mJ of energy with pulses offrom about 20 ns to about 1000 ns; moving the excimer laser relative tothe Si substrate and carbon source; and forming SiC on the Si substrate.17. The method according to claim 16, further comprising forming a layerof the SiC on the Si substrate by moving the excimer laser relative tothe Si substrate until the entire surface of the Si substrate is coveredwith SiC.
 18. The method according to claim 16, wherein the depositing acarbon source on a surface of the Si substrate comprises depositingpolymethyl methacrylate (PMMA) onto a surface of the Si substrate byspin coating.
 19. The method according to claim 16, further comprisingtexturing the SiC formed on the Si substrate by adjusting the pulse rateand focal point of the excimer laser.
 20. The method according to claim6, wherein the method is performed during the manufacture of aphotovoltaic or optical device and further comprising generating carboninterconnects on the SiC by adjusting a laser profile with a shadow masksuch that the power of the laser on its periphery is sufficient to carryout pyrolysis of the carbon source, but low enough that the Si substrateis not melted, and the power of the laser in a center region issufficiently high so that it melts the surface of the substrate anddecomposes the carbon source simultaneously to thereby form SiC.
 21. Aheterojunction device comprising: a Si substrate; and a film of SiCdeposited on a surface of the Si substrate, wherein the SiC has a Si:Cratio that increases or decreases from a SiC surface in contact with theSi substrate to an opposing SiC surface that is not in contact with theSi substrate.